The present invention relates to a new and improved dental prosthesis.
In its more particular aspects the present invention specifically relates to a new and improved dental prosthesis which constitutes a ceramically coated dental prosthesis containing a metal frame and dental ceramic material. Such dental prosthesis may constitute a coated crown, a bridge or the like.
It is a basic prerequisite for alloys which are intended for use in the human body that such alloys must be physiologically safe. Besides the fact which is obvious as such, that the alloy must not dissolve in the human body, a number of further requirements exist when using such alloys in the oral cavity. With respect to the very high corrosive stresses, for example, extreme fluctuations of the pH-value in the oral cavity accompanied by simultaneously variable concentrations of corrosion promoting ions, for example, chloride ions, these alloys still additionally must have properties which are of importance in the course of manufacturing dental prostheses, bridges, or the like. For reasons of appearance as well as of durability, dental prostheses are coated with a ceramic material and this ceramic material is applied to the metal surface by means of firing processes. When firing the ceramic material it must be considered that the temperature is raised by approximately 1,000.degree. C. and, therefore, the differences in the thermal coefficients of expansion may not be too great. Furthermore, if such differences do exist, the thermal expansion of the alloy component must be smaller than that of the ceramic component, so that the ceramic portion is not subjected to tensile stress but to compressive stress after cooling of the dental prostheses because ceramic material possesses only low tensile strength, however, high compressive strength.
Further there often exists the desire that the ceramic material should be applied not by means of a single firing process but by means of a number of firing processes and the behavior during these individual firing processes should correspond to the behavior as initially explained.